<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <script>
      var singleNumber = function (nums) {
        for (let i = 1; i < nums.length; i++) {
          nums[0] ^= nums[i];
        }
        return nums[0];
      };

      console.log(singleNumber([4, 1, 2, 1, 2]));
    </script>
  </body>
</html>
